This invention relates to a method of forming a built-up film, i.e. a film made up of a desired number of monomolecular layers, of a fluorinated aliphatic compound having a hydrophilic group at one terminal on a solid substrate surface by using the Langmuir-Blodgett's technique.
Extensive research and development work has been pursued concerning organic fluorine-containing compounds and fluoro-polymers to utilize their characteristic properties such as hydrophobicity, lubricativeness and high resistance to chemicals in coating or treating various solid surfaces. With the recent advancement of optoelectronics, low indices of refractin of organic fluorine-containing materials are also attracting keen attention.
Polytetrafluoroethylene resin coatings are used for very wide purposes, and some different kinds of fluorine-containing organic coating materials are used for special purposes. For example, a coating material that utilizes coupling reaction of a fluorosilane is used for modification of glass surfaces, and paints containing a perfluorocarboxylic acid are also in practical use. In general, conventinal fluorine-containing organic coatings are formed by application of solution, suspension or paste of the coating materials.
As for coating films on electrical, electronical or optical materials, generally it is desirable to reduce the thickness of the coating films as much as possible from the viewpoint of effectively utilizing the electrical or optical properties inherent to the base materials. In the conventional fluorine-containing organic coatings formed by liquid coating methods there are limits to the reduction of the coating film thickness. Besides, conventional fluorine-containing organic coating films are not always satisfactory in the ability to shield the base material surfaces from intrusion of moisture.
Meanwhile, studies have been made on the formation of built-up films of organic compounds on solid substrate surfaces by using the Langmuir-Blodgett's technique. For example, Clint et al., J. Colloid and Interface Sci., 47, No. 1, 172-185(1974) deal with Langmuir-Blodgett multilayers of cadmium salts of long chain fatty acids and their partially fluorinated derivatives.
In general it is difficult to form built-up films of perfluorohydrocarbons by the usual method because the cohesive energy between adjacent molecules becomes considerably low by the influence of the introduced fluorine. In the cases of fluorohydrocarbons which retain a relatively long hydrocarbon chain, it is possible to obtain built-up films by using the Langmuir-Blodgett's technique. However, the obtained built-up films are not satisfactory in respect of the effects of the introduction of fluorine atoms, such as water repellency, low reflectance, low refraction and high withstand voltage, because of shortness of the fluorocarbon chain compared with the hydrocarbon chain.